1. Technical Field
The present invention relates to a radiation therapy device comprising a radiation source and an aperture plate arrangement located between the radiation source and its object and defining a field of radiation.
2. Description of the Prior Art
U.S. Pat. No. 4,121,109 discloses a radiation therapy device having a aperture plate arrangement in which at least one aperture plate is movable. It has also been proposed to move the plates by a control device during irradiation in such a manner that it is possible to obtain an effective dose distribution that decreases in the open direction of the aperture plates.
From an article "Wedge-Shaped Dose Distribution by Computer-Controlled Collimator Motion" in Medical Physics (5), Sept./Oct. 1978, pages 426 to 429 it is known to use a defined plate motion to obtain a wedge-shaped isodose during irradiation. Such a wedge shaped isodose is frequently desired in radiation therapy in order to adjust to the anatomical conditions of the treatment subject. The wedge-shaped isodose results from the fact that different areas of the radiation field are exposed to irradiation for varying lengths of time. The requisite motion of the plate is caused by an iterative process.
The movable aperture plate can be regarded as a substitute for conventional wedge-shaped filters. It is also possible to obtain a wedge-shaped isodose curve by introducing a wedge-shaped filter between the radiation source and its object; however, in this case the filter has to be changed in accordance with each desired isodose curve.
On the other hand, the movable aperture plates have the disadvantage that the dose always increases in a predetermined direction, namely the one opposite to the opening direction of the plates.